Method of heating underground formations and recovery of oil therefrom



P 1964 M. R. J. WYLLIE 3,147,804

METHOD OF HEATING UNDERGROUND FORMATIONS AND RECOVERY OF 011. THEREFROM Filed Dec. 27, 1960 INVENTOR. 01460: R. .1. WV4L/5 BY ATTORNEY United States Patent METHGD OF HEATHNG UNDERGROUND FORMA- TIONS AND RECOVERY OF @IL THEREFROM Malcolm R. J. Wyllie, Allison Park, Pa., assignor to Gulf Research 5: Development Company, Pittsburgh, Pa, a

corporation of Delaware Filed Dec. 27, 1960, Ser. No. 78,705 8 Claims. (Cl. 166-11) This invention relates to the recovery of oil from underground oil-bearing formations and more particularly to a method of heating an oil-bearing formation to reduce the viscosity of oil therein and facilitate production of the oil.

Frequently, an oil-bearing formation may contain large quantities of oil that cannot be produced at high enough rates to justify the cost of drilling wells and producing the oil. The low rate of production possible from such formations may be caused by loss of pressure on the formation, in which event a secondary recovery process designed to repressure the formation may be successful in increasing the rate of production. Sometimes, particularly in oil-bearing formations usually referred to as tar sands, the oil present in the formation is of such high viscosity that economical rates of production cannot be obtained even though the pressure on the formation is relatively high.

The viscosity of the highly viscous crude oils can be greatly reduced by heating the oils to temperatures moderately higher than the normal formation temperatures. One method that has been proposed to heat the oil-bearing formation is by in situ combustion of a portion of the oil in the formation. The in situ combustion is accomplished by injection of an oxygen-containing gas into the formation and ignition of the oil in the oil-bearing formation. Continued injection of the oxygen-containing gas will cause continued combustion of the oil in the formation with a consequent release of heat and increase in the temperature of the formation.

Two types of in situ combustion processes have been developed. In one, designated forward burning, the combustion front moves in the same direction as the injected oxygen-containing gas and travels from the vicinity of an injection well at which the oxygen-containing gas is displaced into the formation to a production well. In the other type, designated reverse burning, the combustion front is initiated in the vicinity of the production well and travels countercurrently to the injected oxygencontaining gas to the vicinity of the injection well. The forward burning process has a high efficiency caused by heat released by the combustion traveling through cold formations adjacent the production well and heating those formations. Moreover, coke formed ahead of the combustion front as -a result of the high temperature to which the oil is heated is completely burned as the combustion front moves toward the production well to supply the heat for the process and leave a clean sand behind the combustion front. However, particularly when the oil in the oil-burning formation is highly viscous, it is often not possible to inject an oxygen-containing gas at high enough rates to maintain combustion because of the high resistance of the relatively cold oil ahead of the combustion front to flow through the formation.

The reverse burning process can be used where resistance to flow precludes forward burning, but is relatively ineflicient thermally. During the initial stages of the process much of the heat resulting from burning of the oil is discharged through the production well. Moreover, coke formed in the process is not subsequently traversed by the combustion front and, hence, is not burned to supply the heat necessary for the process. If the combustion front has traveled from the vicinity of the production well to the injection well a substantial mass of coke remains in the formation. The coke represents hydrocarbons that cannot be recovered.

One method that has been proposed to overcome the inability to inject an oxygen-containing gas at a high enough rate to maintain combustion in a forward burning process is to form a fracture extending substantially all of the way from the injection well to the production well and thereafter employ a forward burning process in which combustion occurs in the fracture. Many of the tar sands containing highly viscous oils are substantially unconsolidated formations in which a fracture cannot be made and propped open satisfactorily; hence, the combination of a fracture and in situ combustion cannot always be used. Moreover, the fracturing of an oilbearing formation combined with in situ combustion still possesses the disadvantage of consuming oil that is desired to be :produced. Burning oil in the oil-bearing formation also may cause some dam-age to the formation which may seriously impair its permeability.

This invention resides in a process for heating an underground oil-bearing formation in which a substantially impermeable rock stratum adjacent to an oil-bearing formation is fractured from a fuel supply well to an adjacent exhaust well. A fluid fuel and an oxygen-containing gas, usually air, is displaced into the resultant fracture and the mixture ignited in the fracture. Heat supplied by combustion of the fuel is transferred by conduction to the oil-bearing formation. In this invention, the composition of the fuel-oxygen-containing gas mixture injected into the fracture is changed periodically to cause the combustion to alternate between forward and reverse burning whereby the combustion front moves back and forth in the fracture to heat the oil-bearing formation uniformly. The single figure of the drawing is a diagrammatic i1- lustration, partially in vertical section, of a fuel supply and exhaust well arranged for the process of this invention.

O-il reservoirs comprise an oil-bearing formation having some permeability which allows the oil to migrate and collect in quantities sufiicient to permit economical recovery. Overlying the oil-bearing formation is a cap rock which must be substantially impermeable to trap the oil and prevent its escape during the geologic periods prior to drilling wells into the reservoir. Most frequently the cap rock is of argillaceous origin and is of shale or slate. The rocks forming the cap rock may be easily fractured for large distances from a well because of their low permeability and the resultant low leak off of fracturing fluid during a hydraulic fracturing operation. The cap rock usually has well defined bedding planes substantially parallel to the boundary between the cap rock and the oil-bearing formation; hence, it is not difficult to form the fractures of large radial extent in the cap rock near the boundary of the oil-bearing formation.

In the process of this invention, a fracture is made to extend from a fuel supply to an adjacent exhaust well through an impermeable rock stratum adjacent to an oilbearing formation. The fracture, which may be in a stratum either above or below the oil-bearing formation, may be made by fracturing processes designed to orient the fracture substantially parallel to the boundary of the stratum and oil-bearing formation. A preferred method is the method described in United States Patent No. 2,699,212 of Dismukes in which a notch in the desired plane of the fracture is cut in the formation adjacent the borehole of a well and hydraulic pressure is then applied to fracture the rock and form a fracture extending in the plane of the initial notch. The process described in Patent No. 2,699,212 permits the fracture to be accurately located at the desired depth, as well as oriented in the desired plane. It is then possible to place the fracture very near the boundary of the oil-bearing formation and thereby reduce the resistance to conduction of heat to the oil-bearing formation. The fracture is propped open in the usual manner by displacing solid particles such as sand into the fracture.

Heat is supplied to the oil-bearing formation by injecting a fluid fuel-air mixture into the fracture through the stratum and igniting that mixture to cause combustion to proceed in the fracture. The fuel is preferably either a liquid or gaseous hydrocarbon such as a lease gas, natural gas, propane, butane or liquid fuels such as kerosene or fuel oils. In the copending application Serial No. 782,904 of Terwilliger et al., now United States Patent No. 3,097,690, a process is described and claimed for controlling the direction of movement of the combustion front through a permeable formation by controlling the fuel-air ratio. In that patent data are presented showing that a fuel-air ratio greater than 60% of the stoichiometric ratio results in reverse combustion occurring in a permeable formation. Reduction of the fuel-air ratio to below about 40% of the stoichiometric ratio results in forward combustion. By periodically changing the fuel-air ratio, it is possible to change the direction of movement of the combustion front in the fracture to cause the combustion front to traverse back and forth between the injection and production wells and thereby cause uniform heating of the cap rock and the oil-bearin g formation.

Referring to the drawing, an oil-bearing formation is illustrated between an upper cap rock 12 and a bottom stratum 14 of shale. A fuel supply well, indicated generally as 16, is drilled through the oil-bearing formation 10 and into the bottom stratum 14 to a total depth 18. Casing 20 is set to total depth and cemented in accordance with the usual practice.

At a distance from the fuel supply well 16 dependent upon the characteristics of the reservoir, an exhaust well indicated generally by reference numeral 22 is drilled through upper cap rock 12 and oil-bearing formation 10 into the bottom stratum 14. Casing 24 is set in the well 22 to a depth 26 below the boundary of the oil-bearing formation 10 and bottom stratum 14. Thereafter, drilling is resumed and continued until the well 22 extends to a total depth 28. The casing 24 is perforated as indicated at 30 through the oil-bearing formation.

A packer 32 is set in casing 24 between depth 26 and the lowest of perforations 30 and an exhaust tubing 34 is then run through packer 32 for the discharge of products of combustion from exhaust well 22. The packer 32 and exhaust tubing 34 divide well 22 into an annulus section 35 communicating with the oil-bearing formation 10 and a tubing section communicating with the borehole 37 in the bottom stratum 14. The upper end of casing 22 is closed by a suitable upper closure 36 and a side outlet 38 is provided at the upper end of casing 24 for discharging oil produced from the well 22.

A slot 40 is cut in the casing 20 of well 16 at a depth below the depth 26 of the lower end of the casing in well 22. Thereafter, a substantially horizontal notch is cut in the stratum 14. A fracturing fluid is displaced downwardly through the casing 20 and pressure applied through the slot 40 on the bottom cap rock 14. The pressure on the fracturing fluid is increased until the overburden pressure and the strength of the stratum 14 is overcome to form a fracture 42 in the stratum 14. A propping agent, such as sand, is displaced into the fracture 42 to maintain the fracture in an open condition.

Perforations 44 are made in the casing 20 through the oil-bearing formation 10. A packer 46 is then set in the casing between the slot 40 and perforations 44 and an air line 48 is run through the packer. Within the air line is a fuel line 5 supported at its lower end by any suitable arrangement, such as a spider 52, for delivery of fuel into the fracture 42. The annulus between air line 48 and casing 20 is closed by a cap 54 at the upper end of the casing. A side outlet 56 at the upper end of the casing 20 is provided for the discharge of oil produced through well 16. Air line 48 and fuel line 50 are connected with sources of air and fuel, not shown, by lines 58 and 60, respectively, provided with suitable valves 62 and 64 for control of the rates at which the two fluids are delivered into the fracture 42.

The oil-bearing formation 10 is heated by combustion within the fracture 42 of a fuel supplied through the fuel line 50. It is desirable to avoid combustion within the well 16 to avoid exposing the casing 20 to excessively high temperatures. One method of igniting the fuel is to displace a pyrophoric material such as a solution of triethylborane in a liquid hydrocarbon, such as kerosene, down fuel line 50 and into the fracture 42. Displacemerit of the triethylborane is accomplished by means of the fuel which is to supply the heat to the oil-bearing formation 10. After the triethylborane has been displaced into the fracture 42, air is delivered into the fracture through air supply line 48. Immediately upon contact between the air and triethylborane, the triethylborane is ignited and releases sufficient heat to cause ignition of the fuel supplied through fuel line 50. Another method of ignition is to inject a combustible mixture of fuels and oxygen-containing gas into the fracture 42 at the well 16 and ignite the mixture as it enters borehole 37 by means of an electric igniter, not shown, in borehole 37. Fuel is injected at a suitable rate, such as 500,000 standard cubic feet of lease gas per day, and the composition of the fuel-oxygen containing gas mixture is adjusted to cause reverse combustion to proceed through the fracture 42 toward the well 16.

In this invention the relative rates of flow of fuel and air are changed periodically by means of valves 62 and 64 to cause a change in direction of movement of the combustion front in the fracture 42. When the fuel is ignited adjacent the fuel supply well 16, the fuel-air ratio is controlled at less than 30% of the stoichiometric ratio to cause forward burning in the fracture 42 and gradual movement of the combustion front toward the exhaust Well 22. The products of combustion entering the exhaust well 22 through fracture 42 are delivered through exhaust tubing 34 to the atmosphere. When the temperature in the exhaust well 22 indicates that the combustion is approaching the exhaust well, the fuel-air ratio is increased to cause reverse combustion in the fracture 42 whereupon the combustion front moves toward the well 16.

Burning in the fracture 42 is continued to heat, by conduction, oil in the oil-bearing formation 10 to a temperature at which the viscosity of the oil allows the oil to fiow readily through the perforations 30 and 44 of the wells. An increase in temperature of 200 F. to 300 F. may cause a reduction in viscosity to 1% or even 0.1%, depending upon the particular oil, of the viscosity of the oil originally present in the formation and will make possible large increases in production rates. In some instances, the oil in the formation may be at a temperature below the pour point of the oil and it will be necessary to heat the oil to a temperature above the pour point to allow subsequent production at economic rates. Combustion in the fracture can be continued to heat the oil to even higher temperatures but further decreases in viscosity ordinarily do not justify the expense of the additional fuel.

Production of the oil can be obtianed by conventional procedures. In the illustration in the drawing, the oilbearing formation is under adequate Pressure to cause suitable rates of recovery of the oil upon reduction of the viscosity of the oil. The heating process of this invention can, if necesary or desirable, be combined with gas repressuring or other fluid drive processes for moving the oil from the oil-bearing formation 10 into one or both of the wells for lifting to the surface.

The process of this invention can be used advantageously to heat oil-bearing formations containing viscous oil because of the widespread availablity of suitable, easily fractured shales adjacent such formations. Moreover, damage to the oil-bearing formation resulting from hydration of clays or plugging with soot, which may be experienced in the usual in situ combustion process in which a portion of the oil in the oil-bearing formation is burned, can be avoided. Control of the location of the combustion front, and, hence, the peak temperature, allows uniform heating of the formation and thereby reduces heat losses resulting from heating portions of the formation to very high temperatures before other portions are heated to temperatures high enough to cause appreciable reduction in viscosity of the oil.

I claim:

1. A method of heating an oil-bearing formation, penetrated by a fuel supply well and an exhaust well spaced therefrom, having a substantially impermeable stratum of argillaceous origin adjacent thereto, comprising forming a fracture extending in said stratum from the fuel supply well to the exhaust well, depositing a propping agent in the fracture to hold the fracture open, injecting a fluid fuel and oxygen-containing gas mixture into the fracture adjacent the fuel supply well, igniting the fuel within the fracture adjacent the fuel supply well, maintaining the fuel to oxygen-containing gas ratio below about 30% of the stoichiometric ratio to cause forward combustion to proceed in the fracture, thereafter increasing the fuel to oxygen-containing gas ratio to above 60% to cause the combustion to be converted to reverse combustion, and discharging products of combustion through the exhaust well.

2. A process as set forth in claim 1 in which the impermeable stratum is under the oil-bearing formation.

3. A process as set forth in claim 1 in which the impermeable stratum is above the oil-bearing formation.

4. In a recovery process for recovering hydrocarbon fluid from a subterranean oil-bearing formation penetrated by a fuel supply well and an exhaust well, said oilbearing formation having a substantially impermeable stratum adjacent thereto, a fracture extending through the stratum and opening into each of the fuel supply well and exhaust well, and a propping agent in the fracture holding the fracture open, each of said fuel supply well and exhaust well having sealing means positioned therein separating the fracture from the oil-bearing formation and each of said fuel supply well and exhaust well having a tubing string extending through the sealing means defining an annulus above the sealing means, the steps of injecting a fluid fuel and an oxygen-containing gas through one of said annulus and tubing string in the fuel supply well, igniting the fuel in the fracture to form a combustion front in the fracture, continuing the injection of the fuel and oxy-containing gas to burn the fuel in the fracture and thereby heat the oil-bearing formation by conduction, periodically changing the ratio of fuel and oxygen-containing gas injected into the fracture between a ratio exceeding 60% of the stoichiometric ratio and a ratio less than 30% of the stoichiometric ratio to change the direction of movement of the combustion front in the fracture and thereby retain the combustion front in the 6 fracture, discharging products of combustion through one of said annulus and tubing string in the exhaust well, and recovering the oil through the other of said annulus and tubing string in the exhaust well.

5. A process as set forth in claim 4 characterized by the injection of a driving fluid into the oil-bearing formation adjacent the fuel supply well and recovering oil through the exhaust well.

6. A method of heating an oil-bearing formation, penetrated by a fuel supply well and an exhaust well spaced therefrom, having a substantially impermeable stratum of argillaceous origin adjacent thereto, comprising forming a fracture extending in said stratum from the fuel supply well to the exhaust well, displacing a propping agent into the fracture, injecting a fluid fuel and air mixture into the fracture adjacent the fuel supply well, igniting the fuel within the fracture adjacent the exhaust well to form a combustion front within the fracture, maintaining the fuel to air ratio above about 60% of the stoichiometric ratio to cause reverse combustion to proceed in the fracture, thereafter decreasing the fuel to air ratio to below about 30% of the stoichiometric ratio to convert the combustion to forward combustion, discharging products of combustion through the exhaust well, and periodically changing the composition of the fuel-air mixture to cause the direction of movement of the combustion front to change whereby the combustion front remains in the fracture.

7. In a recovery process for recovering hydrocarbon fluid from a subterranean oil-bearing formation penetrated by a fuel supply well and an exhaust well, said oil-bearing formation having a substantially impermeable stratum adjacent thereto, a fracture extending through the stratum and opening into each of the fuel supply well and exhaust well, and a propping agent in the fracture holding the fracture open, each of said fuel supply well and exhaust well having sealing means positioned therein separating the fracture from the oil-bearing formation and each of said fuel supply well and exhaust well having a tubing string extending through the sealing means defining an annulus above the sealing means, the steps of injecting a fluid fuel and air through one of said annulus and tubing string in the fuel supply well, igniting the fuel to form a combustion front in the fracture, continuing the injection of the fuel and air to burn the fuel in the fracture and thereby heat the oil-bearing formation by conduction, discharging products of combustion through one of the annulus and tubing string in the exhaust well, recovering the oil through the other of said annulus and tubing string in the exhaust well, and periodically changing the fuel to air ratio of the gases injected into the fracture between a ratio exceeding 60% of the stoichiometric ratio and a ratio less than 30% of the stoichiometric ratio to change the direction of movement of the combustion front in the fracture and thereby maintain the combustion front in the fracture.

8. A method of heating an oil-bearing formation penetrated by a fuel supply well and an exhaust well spaced therefrom having a substantially impermeable stratum underlying said formation comprising creating a fracture extending through said impermeable stratum from the fuel supply well to the exhaust well, depositing a propping agent in the fracture to hold the fracture open and form a permeable stratum, setting sealing means in each of the fuel supply well and exhaust well between the oilbearing formation and the fracture, running tubing string in each of the fuel supply well and exhaust well through the sealing means to communicate with the fracture, injecting a mixture of fluid hydrocarbon fuel and air into the fracture, said fluid hydrocarbon fuel being gaseous in admixture with the air in the fracture, igniting the fluid hydrocarbon fuel in the fracture to form a combustion front in the fracture, periodically changing the ratio of between a composition having a fuel-air ratio exceeding 60% of the stoichiometric ratio and a composition having a fuel-air ratio less than 30% of the stoichiometric ratio to change the direction of movement of the combustion front in the fracture whereby heat is released in the fracture to heat oil in the adjacent oil-bearing formation, discharging combustion products from the fracture into the tubing in the exhaust Well, and delivering oil from the oil-bearing formation into the annulus in the exhaust Well for delivery to the wellhead.

25 References Cited in the file of this patent UNITED STATES PATENTS Merriam et a1 Feb. 5, 1952 Garrison et al Feb. 3, 1959 OTHER REFERENCES Grant, B. F. and Szaz, S. E.: Development of an Underground Heat Wave for Oil Recovery, Journal of Petroleum Technology, pages 23-33, May 1954.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Noa 3,147,804 September 8 1964 Malcolm R. J, Wyllie It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4 line 39 "fuels" read fuel column 5, line 68, for "oxy-containi 3" read oxygen-containing column 8, strike out lines and insert instead the following:

2 584 605 Merriam et all Feb 5 1952 2,699 212 Dismukes Jan 11, 1955 2,871,9'412 Garrison et a1, Feb. 3 1959 2,917,296 Prentiss Dec, 15 1959 3,09%690 Terwilliger et a1 --Ju1y 16,, 1963 Signed and sealed this 9th day of February 1965.

(SEAL) Attest:

EDWARD J BRENNER Commissioner of Patents ERNEST W0 SWIDER Attesting Officer 

1. A METHOD OF HEATING AN OIL-BEARING FORMATION, PENETRATED BY A FUEL SUPPLY WELL AND AN EXHAUST WELL SPACED THEREFROM, HAVING A SUBSTANTIALLY IMPERMEABLE STRATUM OF ARGILLACEOUS ORIGIN ADJACENT THERETO, COMPRISING FORMING A FRACTURE EXTENDING IN SAID STRATUM FROM THE FUEL SUPPLY WELL TO THE EXHAUST WELL, DEPOSITING A PROPPING AGENT IN THE FRACTURE TO HOLD THE FRACTURE OPEN, INJECTING A FLUID FUEL AND OXYGEN-CONTAINING GAS MIXTURE INTO THE FRACTURE ADJACENT THE FUEL SUPPLY WELL, IGNITING THE FUEL WITHIN THE FRACTURE ADJACENT THE FUEL SUPPLY WELL, MAIN- 